We investigated the dehydrogenation of NaAlH4 and the reversible low-pressure rehydrogenation from NaH to Na3AlH6. Highly purified NaAlH4 requires relatively high temperatures to decompose to NaH and long durations to rehydride to the Na3AlH6 phase in hydrogen gas. However. any degradation of the purity of this material. whether through ball milling with diamond powder or ball milling with diamond Plus Al powders, mixing the purified material with Pt powder, or doping with a Ti organometallic compound, lowers the decomposition temperature and facilitates rehydriding the product NaH+Al to Na3AlH6. Diamond ball milling of NaAlH4 seems to be the best of these procedures; it substantially decreases the decomposition temperatures, with significant dehydrogenation starting at 180 degreesC rather than 250 degreesC for the Purified material, and with formation of NaH substantially complete at 235 degreesC rather than 290 degreesC. Rather surprisingly, it also facilitates rehydrogenation from NaH+Al to Na3AlH6. Similarly, NaAlH4 doped with Ti according to the recipe of Bogdanovie lowers the decomposition temperatures and improves the hydrogenation kinetics for the low pressure transition from NaH+Al to Na3AlH6. Pressure-composition isotherms show that the rehydrogenation of the resulting NaH+Al decomposition phases into the Na3AlH6 intermediate phase at pressures below 3.6 MPa is similar for the diamond ball milled and Ti-doped material. Diamond ball milling NaAlH4 with excess Al did not improve the rehydrogenation kinetics. (C) 2002 Elsevier Science B.V. All rights reserved.
